Production of biodiesel from Jatropha curcas L. oil

Abstract

A two-step process consisting of pre-esterification and transesterification was developed to produce biodiesel from crude Jatropha curcas L. oil. The free fatty acids (FFAs) in the oil were converted to methyl esters in the pre-esterification step using sulfuric acid or solid acid prepared by calcining metatitanic acid as catalysts. The acid value of oil was reduced from the initial 14 mg-KOH/g-oil to below 1.0 mg-KOH/g-oil in 2 h under the conditions of 12 wt% methanol, 1 wt% H₂SO₄ in oil at 70 °C. The conversion of FFAs was higher than 97% at 90 °C in 2 h using 4 wt% solid acid and a molar ratio of methanol to FFAs of 20:1. Phospholipid compounds were eliminated during pre-esterification and a separate degumming operation was unnecessary. The yield of biodiesel by transesterification was higher than 98% in 20 min using 1.3% KOH as catalyst and a molar ratio of methanol to oil 6:1 at 64 °C.

Introduction

Biodiesel is an alternative fuel produced from renewable vegetable oils, animal fats or recycled cooking oils by transesterification reaction. Biodiesel has drawn significant attention due to increasing environmental concern and diminishing petroleum reserves (Ma & Hanna, 1999). Presently, biodiesel is produced commercially in Europe and USA to reduce air pollution and the net emission of greenhouse gas. Surplus edible oils, such as rapeseed oil and soybean oil, are used as raw materials for biodiesel (Körbltz, 1999, Wood, 2005). However, using edible oils to produce biodiesel is not encouraged in China because China imports more than 400 million tons of edible oils annually to satisfy its consumption needs. Some Chinese biodiesel producers use recycled waste oils to produce biodiesel, but the scale is limited. Although the use of waste oils can lower the feedstock cost significantly, complicated procedures are needed to remove the impurities, resulting in high operating costs (Al-Widyan & Al-Shyoukh, 2002; van Kasteren & Nisworo, 2007; Zhang, Dubé, McLean, & Kates, 2003a; Zhang, Dubé, McLean, & Kates, 2003b). Non-edible oils like Jatropha curcas L. oil are attractive (Foidl, Foidl, Sanchez, Mittelbach, & Hackel, 1996; Mohibbe Azam, Waris, & Nahar, 2005; Sarin, Sharma, Sinharay, & Malhotra, 2007; Tiwari, Kumar, & Raheman, 2007; Wood, 2005). J. curcas L. trees can grow in arid, semiarid and wastelands. It has a high-seed yield and high oil content (Wood, 2005). In China, its plantation area is being expanded quickly along the Yangzi River as promoted by an environment protection act.

In conventional processes, biodiesel is manufactured by the transesterification of oils with methanol in the presence of catalysts, such as alkalis (KOH, NaOH) or their corresponding alkoxides (Freedman, Pryde, & Mounts, 1984; Holser & Harry-O’Kuru, 2006; Ikwuagwu, Ononogbu, & Njoku, 2000; Jitputti et al., 2006; Leung & Guo, 2006; Ma & Hanna, 1999; Siler-Marinkovic & Tomasevic, 1998):

triglycerides + methanol → biodiesel + glycerol

The process design and operation parameters vary with the properties of the feedstock oils and the desired biodiesel quality. Commercial biodiesel processes using rapeseed oil (in Europe) and soybean oil (in the USA) have been well researched, and thereby the properties of their biodiesel products have also been comprehensively investigated.

However, J. curcas L. oil with high content of free fatty acids (FFAs) cannot be directly used in an alkali catalyzed transesterification process because FFAs react with alkali catalyst to form soaps, resulting in serious emulsification and separation problems. Pre-esterification catalyzed by homogeneous acids, such as sulfuric acid, phosphorous acid, or sulfonic acid, is a conventional and useful method to reduce the content of FFAs, which can turn the raw oils transesterificable by an alkali catalyst and convert FFAs to valuable fatty acid methyl esters (FAME) (Ghadge & Raheman, 2005; Tiwari et al., 2007). Compared with conventional liquid acid catalysts, solid acid catalyst is more environmentally friendly (López, Suwannakarn, Bruce, & Goodwin, 2007; Mbaraka & Shanks, 2005; Narasimharao et al., 2007). The effect of methanol to oil ratios on FFA conversion at the reaction temperature of 50 °C, a reaction time of 1 h, and a H₂SO₄ to oil ratio of 1% (w/w) has been investigated (Berchmans & Hirata, 2008). The optimum methanol to oil ratio was found to be 60% (w/w) for an FFA concentration less than 1%, and an acid value (AV) of 2 mg-KOH/g-oil.

We propose a two-step method to convert raw J. curcas L. oil into biodiesel. A pre-esterification operation was applied to eliminate FFAs by reacting the oil with methanol in the presence of an acid catalyst. The process can be simply described as

pre-esterificatin → purification → transesterification → phase separation

Raw oil was firstly reacted with methanol, followed by phase separation to remove acidic water and gum impurities. The purified oil was further reacted with methanol in Section 2.4 in the presence of an alkali catalyst. Finally, the biodiesel product was separated from the glycerol by-product by phase separation.

In this work, factors influencing the pre-esterification and transesterification were systematically investigated, and the solid acid catalyst SO₄²⁻/TiO₂ prepared by calcining metatitanic acid was tested and characterized. The properties of the biodiesel were measured.